A striking characteristic of many different eukaryotic transcriptional activators is their ability to activate gene expression synergistically. Thus, for example, the rat glucocorticoid receptor and the yeast activator GAL4 cooperatively activate transcription of a mammalian gene bearing binding sites for each of the proteins: activation by both activators is greater than the sum of the effects of each working alone. It would seem unlikely that these two proteins from such different organisms directly interact; rather, the idea has been suggested that these and at least some other eukaryotic activators can work synergistically by simultaneously touching some part of the transcriptional machinery. An important prediction of this idea is that synergy between two such activators would be seen under conditions where each is present at concentrations sufficiently high to saturate its site on DNA. In this paper we use transcription in vitro to confirm that prediction using a derivative of the yeast activator GAL4 and the mammalian transcription factor ATF. The accompanying paper describes a similar conclusion comparing the effects of singly and multiply bound GAL4 molecules.